7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

MHD and energetic ions instabilities related to the formation of ITBs in EAST
reversed shear plasmas with 𝒒𝒎𝒊𝒏 ≃ 𝟐
Ming Xu1, on the behalf of EAST team1
Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP)
e-mail (speaker): mxu@ipp.ac.cn

The formation of internal transport barriers (ITBs) has
been investigated in EAST weak/reversed shear plasmas
with minimum safety factor 𝑞𝑚𝑖𝑛 ≤ 2. The so-called“off-axis” sawteeth (OAS) is frequently excited for thereversed shear magnetic configuration, which triggeredby the magnetic reconnection of m/n=2/1 double tearing
modes (DTMs) [1], where m, n are the poloidal andtoroidal mode numbers respectively. Abundant ofphenomena are easily excited during the oscillation ofOAS, e.g., two kinds of Alfven Eigenmodes (BAEs andRSAEs) are excited by the beam ions generated from
NBI [2-3], and two kinds of low-frequency modes(LFMs) are excited by thermal pressure gradient [4-5], etal. The suppression of OAS or DTM is one important
step to establish the structure of ITBs. All those results
listed above are summarized in Fig.1.
One example for the suppression of OAS or DTM isdemonstrated in Fig. 2. The reversed magnetic
configuration is established by LHCD and ECRH, andthe oscillation period of OAS is 𝜏𝑂 ≈ 100𝑚𝑠 for theEAST shot #61959. In shot #61962, the 𝜏𝑂 ≈ 260𝑚𝑠for the perpendicular injection of NBI (NBI1R), which is
fully suppressed for the tangential injection of NBI(NBI1L), as marked by the yellow shaded region.
The establishment (or sustainment) of ITBs is often
accompanied by the Alfven eigenmodes. Similarly,
several different instabilities can be observed during theformation of ITBs in EAST, e.g., Alfven ion temperature
gradient (AITG) mode is excited by thermal pressure
gradient, BAEs and RSAEs are excited by energetic ions,
GAM instability is related to the poloidal flow, et al. One
typical example of the formation of ITBs are selected asgiven in Fig. 3. The two similar discharge conditions forthe case without and with OAS are given by the blue andred lines respectively. The existence of OAS is an
important excitation condition for the establishment ofITBs, and the structure of ITBs forms gradually when thepower of NBI with 𝑃𝑁𝐵 ≈ 4𝑀𝑊 is injected.
Interestingly, the fast ions density decreased by 3% for
the excitation of the pairs of BAEs-RSAEs instabilities.
The redistributed distribution of fast ions is beneficial forthe formation of ITBs, and some possible physics arediscussed.
References[1] Ming Xu, et al. Nucl. Fusion 61 (2021) 106008.
[2] Ming Xu, et al. Nucl. Fusion 59 (2019) 084005.
[3] Ming Xu, et al. Chin. Phys. Lett. 38 (2021) 085201.
[4] Ming Xu, et al. Nucl. Fusion 60 (2020) 112005.
[5] Ming Xu, et al. Nucl. Fusion, 62 (2022) 126041.
Figure 1. The ITBs can be excited by suppression ofDTMs, and fast ions or pressure gradient induced
instabilities are observed for the condition of 𝑞𝑚𝑖𝑛 ≤ 2in EAST.

Figure 2. The oscillation period of OAS is suppressed
dramatically by the tangential injection of NBI.

Figure 3. One typical example for the formation of ITBs
together with the excitation of the pairs of BAEs-RSAEs
instabilities. (a) plasma stored energy for two similarcases, (b) neutron yield 𝑆n , (c) spectrogram of ECE
signal at R=2.06 m (0.4 ≤ 𝜌 < 0.5).